1. Field of the Invention
The present invention relates to a process for preparing a porous hybrid comprising zeolite and chitosan and a porous hybrid prepared thereby, in particular, to a process for preparing a porous hybrid capable of simultaneously removing both cation and anion, a porous hybrid prepared thereby, an adsorbent and a method for purifying water.
2. Description of the Related Art
Zeolite is a generic name of crystalline aluminosilicate, which constitutes the pore skeleton of zeolite molecules and bears an anionic charge for each aluminum atom. Cations for offsetting such anion charges are present within the pore space and the remaining pore space is filled with water. The three-dimensional pore structure of the zeolite molecule varies depending on the shape and size of the pore, and the pore diameter is usually determined by size of the molecule. Therefore, based on the shape and size of the pore, zeolite has the size and shape selectivity for molecules entering into the pore. In this connection, zeolite is called a molecular sieve.
Meanwhile, there are many known zeotype molecular sieves wherein a part or all of silicon (Si) and/or aluminum (Al) atoms constituting the structural skeleton of zeolite molecule are replaced with other elements. For example, a mesoporous silica (MCM-series mesoporous silica and silicate) in which aluminum atoms are completely eliminated, an alpo (AlPO4)-typed molecular sieve in which silicon atoms are replaced with phosphorous atoms, and other molecular sieve or zeotype material wherein skeleton metal atoms are partially replaced with various metal atom such as Ti, Mn, Co, Fe and Zn have been developed and widely used. The materials described above are derived from zeolites and thus generally called as zeolites in the art, while they not belong to zeolites in terms of mineralogy. Accordingly, the term “zeolite” used herein refers to zeolite in a broad sense including zeotype molecular sieves described previously.
Zeolites are widely used in the field of households and various industries serving as a catalyst for cracking petroleum, adsorbent, water-absorbing agent, gas-purifying agent, additives for detergent and soil improving agent. In particular, zeolites are very useful as an ion exchanger to eliminate heavy metals, radioisotopes and diverse ionic dyes in industrial wastewater.
Such molecular sieves such as zeolite or zeotype materials are prepared by crystallizing the precursor thereof and generally obtained in the form of fine powder. However, the find powder form gives rise to some serious shortcomings in application of zeolites.
For example, when such zeolite or zeotype materials in the form of powder are charged into a fixed bed, it is difficult for a liquid or gaseous fluid to flow through the powder since the pressure drop phenomena severely occurs. Therefore, a very high pressure is required to maintain a sufficient flow velocity in the fixed bed charged with molecular sieve powder, which causes some problems such as much energy consumption. Accordingly, there have been proposed various countermeasures in order to avoid such problems owing to the pressure drop phenomena.
The most commonly known method is to prepare a zeolite-clay composite in which zeolite powder is conglomerated using clay as a binder to form a paste, which is then granulated to pellets or beads with the size of 2-5 mm. However, the above-described method the inorganic binder interferes the access of ions to be removed into the inner portion of the pellets or beads, so that the efficiency of zeolite in ion exchange becomes lower. In addition, pellets or beads come untied when exposed to water for a long period of time to form fine powders that are difficult to remove.
Chitosan is the most abundant biopolymer second only to cellulose (1). It is generally prepared by deacetylating chitin found in a crustacean such as crab and lobster, insects such as grasshopper and dragonfly, mushrooms such as enoki mushroom and black mushroom, and bacteria. Chitin is annually produced in the amount of about a hundred billion tons.
Chitosan is a polysaccharide consisting of N-acetyl-D-glucosamine residues in β-1,4 linkage, which is lack of acetyl groups in amine groups found in chitin contributing to the existence of polycation in acidic solution. Therefore, chitosan shows considerable water-solubility in aqueous acid solution and therefore exhibits excellent workability and physical strength after drying. In this connection, chitosan is usually prepared in the form of powder, fiber, film, gel or bead (2). Depending on the number of monomer, chitosan is classified to oligomeric chitosan having around 12 monomers and polymeric chitosan. The polymeric chitosan is in turn classified to a low-molecular weight chitosan with MW of below 150,000, a high-molecular weight chitosan with MW of from 700,000-1,000,000 and a middle-molecular weight chitosan with intermediate MW range.
Chitosan has been reported to excellent adsorption capacity to anionic pigments and heavy metals (3-6), in which amine groups in glucosamine residues form complex ions through coordinate bonds to capture heavy metals in wastewater. More advantageously, chiotosan exhibits remarkable adsorption capacity to low concentration of transition metal ions in Period 3. However, chitosan is susceptible to solubilization in water, in particular, acidic medium (15), which renders the removal of chitosan difficult in processes using aqueous medium.
To be free from the shortcomings described previously, chitosan beads prepared using basic solution are treated with glutaraldehyde for crosslinking chitosan polymers so as to prevent solubilization in water and the resultants serve as filler in fixed bed typed filter (16). However, the crosslinkages formed between chitosan polymer stands significantly decrease availability of the inner portion of chitosan beads to access of ions as inorganic binder, so that the inner portion of chitosan beads treated with glutaraldehyde is not available in adsorption.
In the meantime, there have not been yet reported ion exchangers capable of removing both harmful cations such as heavy metal ions and harmful anions in wastewater. In this regard, it is general in the art that two types of ion exchanger capable of removing each of anion and cation are mixed merely through physical methods in order to simultaneously remove harmful cations and anions in wastewater. However, even if zeolite and chitosan follow such approach, they cannot be free from the drawbacks described hereinabove.
Throughout this application, various patents and publications are referenced and citations are provided in parentheses. The disclosure of these patents and publications in their entities are hereby incorporated by references into this application in order to more fully describe this invention and the state of the art to which this invention pertains.